This work is a systematic investigation of growth of a mammalian fetus. It consists of three (3) parts: a) Description of normal rates of intrauterine growth of individual sheep. b) Relationship between specific rates of growth and roles of utilization of specific metabolic substrates. c) Description of the mechanisms whereby the fetus obtains the necessary metabolites - specifically the role of steroid hormones in development of the placenta and regulation of the uterine circulation.